diff --git a/backend/src/db.ts b/backend/src/db.ts
index da68752..98e6591 100644
--- a/backend/src/db.ts
+++ b/backend/src/db.ts
@@ -7,41 +7,61 @@ mkdirSync(dirname(dbPath), { recursive: true })
 
 export const db = new DatabaseSync(dbPath)
 
-// Cache of email -> Slack avatar URL, so we don't hit Slack on every request.
-// image_url is nullable: a NULL row means "looked up, no Slack match" (also
-// worth caching, to avoid re-querying people who aren't in Slack).
+// A snapshot of the Slack workspace directory: one row per Slack user that has
+// an email, plus a single meta row recording when the snapshot was last swept.
+// An unmatched Member is simply one whose email isn't in this table (see ADR-0001).
 db.exec(`
-  CREATE TABLE IF NOT EXISTS slack_avatar_cache (
+  CREATE TABLE IF NOT EXISTS slack_directory (
     email TEXT PRIMARY KEY,
     image_url TEXT,
-    cached_at INTEGER NOT NULL
-  )
+    slack_id TEXT
+  );
+  CREATE TABLE IF NOT EXISTS slack_directory_meta (
+    id INTEGER PRIMARY KEY CHECK (id = 1),
+    refreshed_at INTEGER NOT NULL
+  );
 `)
 
-const CACHE_TTL_MS = 7 * 24 * 60 * 60 * 1000
-
-interface CacheRow {
-  image_url: string | null
-  cached_at: number
+export interface DirectoryEntry {
+  email: string
+  imageUrl: string | null
+  slackId: string
 }
 
-// Returns the cached URL (which may be null = known-no-match), or undefined
-// when there is no fresh cache entry and the caller should query Slack.
-export function getCachedAvatar(email: string): string | null | undefined {
+export function getDirectoryRefreshedAt(): number | null {
   const row = db
-    .prepare("SELECT image_url, cached_at FROM slack_avatar_cache WHERE email = ?")
-    .get(email.toLowerCase()) as CacheRow | undefined
-  if (!row) return undefined
-  if (Date.now() - row.cached_at > CACHE_TTL_MS) return undefined
-  return row.image_url
+    .prepare("SELECT refreshed_at FROM slack_directory_meta WHERE id = 1")
+    .get() as { refreshed_at: number } | undefined
+  return row ? row.refreshed_at : null
 }
 
-export function setCachedAvatar(email: string, imageUrl: string | null): void {
-  db.prepare(
-    `INSERT INTO slack_avatar_cache (email, image_url, cached_at)
-     VALUES (?, ?, ?)
-     ON CONFLICT(email) DO UPDATE SET
-       image_url = excluded.image_url,
-       cached_at = excluded.cached_at`,
-  ).run(email.toLowerCase(), imageUrl, Date.now())
+// Atomically replace the whole directory snapshot and stamp refreshed_at.
+export function replaceDirectory(entries: DirectoryEntry[]): void {
+  const insert = db.prepare(
+    "INSERT OR REPLACE INTO slack_directory (email, image_url, slack_id) VALUES (?, ?, ?)",
+  )
+  const stamp = db.prepare(
+    "INSERT OR REPLACE INTO slack_directory_meta (id, refreshed_at) VALUES (1, ?)",
+  )
+  db.exec("BEGIN")
+  try {
+    db.exec("DELETE FROM slack_directory")
+    for (const e of entries) insert.run(e.email.toLowerCase(), e.imageUrl, e.slackId)
+    stamp.run(Date.now())
+    db.exec("COMMIT")
+  } catch (err) {
+    db.exec("ROLLBACK")
+    throw err
+  }
+}
+
+export function getDirectoryMap(): Map<string, DirectoryEntry> {
+  const rows = db
+    .prepare("SELECT email, image_url, slack_id FROM slack_directory")
+    .all() as { email: string; image_url: string | null; slack_id: string }[]
+  const map = new Map<string, DirectoryEntry>()
+  for (const r of rows) {
+    map.set(r.email, { email: r.email, imageUrl: r.image_url, slackId: r.slack_id })
+  }
+  return map
 }
diff --git a/backend/src/index.ts b/backend/src/index.ts
index 639cced..2b3f2e5 100644
--- a/backend/src/index.ts
+++ b/backend/src/index.ts
@@ -1,10 +1,10 @@
 import { serve } from "@hono/node-server"
 import { Hono } from "hono"
 import { logger } from "hono/logger"
-import { config, hasNaptaCredentials } from "./config.ts"
+import { config, hasNaptaCredentials, hasSlackCredentials } from "./config.ts"
 import { fixtureMembers, fixtureProjects } from "./fixtures.ts"
 import { fetchNaptaProjectMembers, fetchNaptaProjects } from "./napta.ts"
-import { enrichWithSlackAvatars } from "./slack.ts"
+import { enrichWithSlackAvatars, refreshDirectory } from "./slack.ts"
 import "./db.ts" // initialise the SQLite schema at boot
 
 function errorMessage(err: unknown): string {
@@ -32,17 +32,31 @@ app.get("/api/projects", async (c) => {
 app.get("/api/projects/:id/members", async (c) => {
   const id = c.req.param("id")
   if (!hasNaptaCredentials) {
-    return c.json({ source: "fixture", members: fixtureMembers(id) })
+    return c.json({ source: "fixture", slack: "unconfigured", members: fixtureMembers(id) })
   }
   try {
-    const members = await enrichWithSlackAvatars(await fetchNaptaProjectMembers(id))
-    return c.json({ source: "napta", members })
+    const { members, slack } = await enrichWithSlackAvatars(
+      await fetchNaptaProjectMembers(id),
+    )
+    return c.json({ source: "napta", slack, members })
   } catch (err) {
     console.error("fetchNaptaProjectMembers failed:", err)
     return c.json({ error: errorMessage(err) }, 502)
   }
 })
 
+// Force a re-sweep of the Slack directory (the manual cache invalidation).
+app.post("/api/slack/refresh", async (c) => {
+  if (!hasSlackCredentials) return c.json({ error: "Slack is not configured" }, 400)
+  try {
+    const refreshed = await refreshDirectory()
+    return c.json({ refreshed })
+  } catch (err) {
+    console.error("slack refresh failed:", err)
+    return c.json({ error: errorMessage(err) }, 502)
+  }
+})
+
 console.log(
   `photofetch backend listening on :${config.port} (napta: ${hasNaptaCredentials})`,
 )
diff --git a/backend/src/slack.ts b/backend/src/slack.ts
index e81c4e3..9be00a2 100644
--- a/backend/src/slack.ts
+++ b/backend/src/slack.ts
@@ -1,9 +1,99 @@
-import type { Member } from "./types.ts"
+import { config, hasSlackCredentials } from "./config.ts"
+import type { Member, SlackStatus } from "./types.ts"
+import {
+  type DirectoryEntry,
+  getDirectoryMap,
+  getDirectoryRefreshedAt,
+  replaceDirectory,
+} from "./db.ts"
 
-// Given Napta members (which carry emails), fill in imageUrl from Slack by
-// matching on email — caching results in slack_avatar_cache (see db.ts).
-// Implemented in Step 11; for now it's a no-op pass-through so the pipeline
-// is wired end-to-end and members render with initials fallbacks.
-export async function enrichWithSlackAvatars(members: Member[]): Promise<Member[]> {
-  return members
+const TTL_MS = 30 * 24 * 60 * 60 * 1000 // 30 days
+
+interface SlackUser {
+  id: string
+  deleted?: boolean
+  is_bot?: boolean
+  profile?: {
+    email?: string
+    image_512?: string
+    image_192?: string
+    image_72?: string
+  }
+}
+
+interface UsersListResponse {
+  ok: boolean
+  error?: string
+  members?: SlackUser[]
+  response_metadata?: { next_cursor?: string }
+}
+
+async function slackUsersList(cursor?: string): Promise<UsersListResponse> {
+  const url = new URL("https://slack.com/api/users.list")
+  url.searchParams.set("limit", "200")
+  if (cursor) url.searchParams.set("cursor", cursor)
+  const res = await fetch(url, {
+    headers: { Authorization: `Bearer ${config.slackBotToken}` },
+  })
+  if (!res.ok) throw new Error(`Slack users.list HTTP ${res.status}`)
+  const body = (await res.json()) as UsersListResponse
+  if (!body.ok) throw new Error(`Slack users.list error: ${body.error}`)
+  return body
+}
+
+// Sweep the whole workspace (paginated) and replace the cached directory.
+// Skips deleted users and bots. Returns the number of entries stored.
+export async function refreshDirectory(): Promise<number> {
+  const entries: DirectoryEntry[] = []
+  let cursor: string | undefined
+  do {
+    const page = await slackUsersList(cursor)
+    for (const u of page.members ?? []) {
+      if (u.deleted || u.is_bot || u.id === "USLACKBOT") continue
+      const email = u.profile?.email
+      if (!email) continue
+      const imageUrl =
+        u.profile?.image_512 || u.profile?.image_192 || u.profile?.image_72 || null
+      entries.push({ email, imageUrl, slackId: u.id })
+    }
+    cursor = page.response_metadata?.next_cursor || undefined
+  } while (cursor)
+  replaceDirectory(entries)
+  return entries.length
+}
+
+async function ensureDirectoryFresh(): Promise<void> {
+  const refreshedAt = getDirectoryRefreshedAt()
+  if (refreshedAt == null || Date.now() - refreshedAt > TTL_MS) {
+    await refreshDirectory()
+  }
+}
+
+export interface EnrichResult {
+  members: Member[]
+  slack: SlackStatus
+}
+
+// Fill each Member's Avatar from the cached Slack directory, matched by email.
+// Degrades gracefully: a Slack failure never hides the roster.
+export async function enrichWithSlackAvatars(members: Member[]): Promise<EnrichResult> {
+  if (!hasSlackCredentials) {
+    return { members, slack: "unconfigured" }
+  }
+  let slack: SlackStatus = "ok"
+  try {
+    await ensureDirectoryFresh()
+  } catch (err) {
+    console.error("Slack directory refresh failed:", err)
+    slack = "degraded" // fall through and match against whatever snapshot exists
+  }
+  const dir = getDirectoryMap()
+  if (dir.size === 0) {
+    return { members, slack: "degraded" }
+  }
+  const matched = members.map((m) => {
+    const hit = dir.get(m.email.toLowerCase())
+    return hit?.imageUrl ? { ...m, imageUrl: hit.imageUrl, slackMatched: true } : m
+  })
+  return { members: matched, slack }
 }
diff --git a/backend/src/types.ts b/backend/src/types.ts
index ef4ffa3..b55d397 100644
--- a/backend/src/types.ts
+++ b/backend/src/types.ts
@@ -15,3 +15,7 @@ export interface Member {
 }
 
 export type DataSource = "napta" | "fixture"
+
+// ok = matched against a fresh directory; degraded = Slack unreachable, matched
+// against a stale/empty snapshot; unconfigured = no Slack token set.
+export type SlackStatus = "ok" | "degraded" | "unconfigured"